Flat-panel displays are of great interest as information displays for computing, entertainment, and communications. For example, electroluminescent (EL) emitters have been known for some years and have recently been used in commercial display devices. Such displays typically employ a plurality of subpixels disposed over a display substrate. Each subpixel contains an EL emitter and, in active-matrix control schemes, a drive transistor for driving current through the EL emitter. The subpixels are typically arranged in two-dimensional arrays with a row and a column address for each subpixel, and having a data value associated with the subpixel. Single EL subpixels can also be employed for lighting and user-interface applications. EL subpixels can be made using various emitter technologies, including coatable-inorganic light-emitting diode, quantum-dot, and organic light-emitting diode (OLED). A typical EL subpixel includes an anode, one or more light-emitting layers, and a cathode.
However, EL emitters suffer from faults that can render an emitter defective, causing so-called “dim dots,” which do not emit as much light for a given drive current or voltage as their neighbors, or “dead dots,” which emit substantially no light. For example, shorts between the anode and cathode of an emitter can provide current paths that bypass the light-emitting layers. Moisture ingress into the light-emitting layers can damage or destroy the light-emitting properties of those layers. Manufacturing faults in the substrate or drive transistor can damage or open the connection between the drive transistor and the EL emitter. Detection of dim or dead dots is an important step in the manufacturing process, both to avoid shipping defective panels and to provide opportunities to compensate for the detected dim or dead dots, and continues to be important as faults develop over the life of a display.
Various schemes compensate for image variation due to defective emitters. For example, US Patent Application Publication No. 2007/0126460 to Chung et al. describes inspecting a panel during fabrication to determine the location of defects and electrically connecting a normal pixel to the defective pixel to compensate. However, this scheme is expensive and time-consuming. It requires laser-welding adjacent EL emitters together, which degrades image quality. Moreover, it cannot compensate for failures due to moisture ingress, which occur periodically over the life of the display.
Commonly-assigned US Patent Application Publication No 2006/0164407 to Cok teaches various methods for compensating for defective subpixels. However, this disclosure teaches measuring the light output of each subpixel to determine which subpixels are defective. This is very difficult to do except in controlled manufacturing conditions. Therefore, failures over the life of the display can only be compensated for by special equipment duplicating those manufacturing conditions.
U.S. Pat. No. 7,474,115 to Trujillo et al. teaches measuring a display device using an infrared camera and suffers from the same limitations as the disclosure of Cok.
US Patent Application Publication No. 2006/0256048 to Fish et al. teaches using a photodiode in each subpixel to measure the light output of the subpixel and compensate for variations in the emitter. However, this scheme requires a very complex subpixel circuit, reducing the area available to emit light and therefore increasing the power and reducing the lifetime of a display, and reducing the manufacturing yield of functional displays.
U.S. Pat. No. 6,965,395 to Neter teaches various ways of compensating for defective pixels in a CCD or CMOS image sensor. However, this method relies on filtering incoming sensed data, and therefore requires the incoming data not have high-frequency, high-amplitude edges that can be confounded with defects. However, such edges are common in display applications, and are found, for example, at the edges of characters in the display of a word processing program, or at the edge of a ticker at the bottom of the screen on a television program.
There is a continuing need, therefore, for a method for detecting defective pixels over the life of an electroluminescent display which is optimized for use in displays and does not require complex equipment or display electronics.